Four power system parameters—frequency,
amplitude, waveform, and symmetry—can serve as frames of reference to classify
the voltage and power disturbances according to their impact on the quality of
the normal sine wave of system voltage.

A brief discussion is given below of the
need for evaluation of their impact on sensitive loads.

a) Frequency variations are rare on
utility-connected systems, but engine-generator based distribution systems can
experience frequency variations due to load variations and equipment
malfunctions.

b) Amplitude variations can occur in
several forms; their description is inextricably associated with their
duration. They range from extremely brief duration to steady state conditions,
making the description and definition difficult, even controversial at times.

Their causes and effects need close
examination to understand the mechanisms and to define an appropriate solution.

c) Waveform variations occur when nonlinear
loads draw a current that is not sinusoidal. One could also describe an
amplitude variation as momentary waveform variation, but the intended meaning
of the term is a steady variation of the waveform, or lasting at least over
several cycles.

This type of disturbance may be described
as harmonic distortion because it is easy to analyze as the superposition of
harmonics to the nominal frequency of the power system.

d) Dissymmetry, also called unbalance,
occurs when unequal single-phase loads are connected to a three-phase system
and cause a loss of symmetry. This type of disturbance primarily concerns
rotating machines and three-phase rectifiers and, as such, is not receiving
broad attention. It is important, however, for machine designers and users.

The
percentage by which one-phase voltage differs from the average of all three is
the usual description of this type of disturbance. A detailed definition of
various measures of voltage and power quality by magnitude, duration, and
spectral content is provided in IEEE Std 1159-1995.

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